[Holmz]

... It sounds terrible (i think clipping), but I found this out only after purchasing new speakers (PIONEER TS-G1720F).
If I purchase this amp (PIONEER GM-A3702) will it amplify the bad signal recieved from the stereo or will it just improve the sound quality overall?
...

^My favourite topic^
(Troubleshooting)

If it sound bad because of the amplification, then a better amp would help.
Maybe some ear buds into the RCAs will tell you if the signal is sounding bad before the amp.
If the signal is bad then it can only get worse down the line, so start with the HU and see/hear where the distortion creeps in.